The present invention relates to a motor having an axial magnetic gap, and more particularly to a motor adapted to be used in a disk record player or a magnetic tape recorder in which the rotating speed of a rotor magnet is sensed to control the rotating speed of the rotor magnet.
In the motor used in the disk record player which plays back a disk record and the magnetic tape recorder/player which records a signal on a magnetic tape and plays it back, it is required to rotate a turntable and a pinch roller at constant speeds and hence it is necessary to control the rotating speed of the motor.
In order to maintain the rotating speed of the motor constant, the rotating speed of the motor may be converted to a voltage signal or frequency signal, which is then compared with a reference voltage or frequency to increase or decrease the rotating speed of the rotor in accordance with a difference between the signals.
A motor has been proposed, which uses a multi-pole magnet as the rotor and a velocity detecting coil arranged in a magnetic field of the multi-pole magnet to interlink with magnetic fluxes of the multi-pole magnet so that the rotating speed of the multi-pole magnet is converted to an A.C. signal. Such a motor is disclosed in detail in Japanese Patent Application Laid-Open No. 51510/77, laid open on Apr. 25, 1977 (corresponding to Japanese Patent Application No. 127768/75, U.S. Patent Application Ser. No. 735,032 filed on Oct. 22, 1976, now U.S. Pat. No. 4,093,897 and German Patent Application No. P2647675.5 filed on Oct. 21, 1976). This motor has a disk magnet attached to a rotary shaft, which disk magnet is magnetized in the direction of the thickness and circumferentially segmented into eight poles with each pole being magnetized in opposite polarity to adjacent poles. A pair of drive coils wound in a star-shape are arranged to face the multi-pole magnet and the velocity detecting coil is arranged to face the multi-pole magnet between the drive coils and the multi-pole magnet. In this motor, the pair of drive coils and the velocity detecting coil are fixed and a magnetic gap is provided in the direction of the axis around which the multi-pole magnet rotates. The velocity detecting coil includes a plurality of radially extending series-connected generating element wires, each of which interlinks with the magnetic flux of each magnetic pole of the multi-pole magnet to produce the velocity detecting signal having a frequency representative of the rotating speed of the multi-pole magnet. However, since the velocity detecting coil interlinks with the magnetic fluxes of the eight magnetic poles of the multi-pole magnet to produce the velocity signal, the frequency of the velocity signal is too low. For example, when this motor is used in the disk record player to directly drive the turntable, the velocity signal has a frequency of 20 Hz if the number of the generating element wires of the velocity detecting coil is equal to 72 and the rotating speed of the turntable is equal to 331/3 r.p.m. Accordingly, a response speed of a velocity control circuit which processes the velocity signal is slow. The velocity control circuit converts the velocity signal to a D.C. voltage which ia representative of the frequency of the velocity signal and compares the D.C. voltage with a reference voltage from a reference voltage source and controls a drive current supplied to the drive coils in a manner to reduce the difference between those voltages. However, since the frequency of the velocity signal is low, a time contant of a low-pass filter to which the velocity signal is fed is large and hence the response speed of the velocity control circuit is low.
In order to raise the frequency of the velocity signal, the number of the generating element wires of the velocity detecting coil may be increased. In this case, however, a space between adjacent element wires becomes so narrow that signals may be produced simultaneously in the adjacent element wires, resulting in the reduction of the output voltage of the velocity signal.